A great game, provided it is taken as it is.
That said, control-wise, Sonic 4 does not feel like its predecessors. This does not mean, however, that the game is bad (in fact, it's quite the opposite). Where in the past holding forward would bring a fast acceleration by itself, now you'll often find yourself starting off a run with an air dash in order to replicate the same sort of acceleration. Additionally, acceleration during the release of holding forward has changed a bit, and requires a minimum speed before Sonic will just keep going instead of stopping himself. For those that have played the originals to death, this will probably result in a lot of lost lives. However, to those who can adapt to it (and it really isn't that hard to) the controls now actually allow you to be a lot more precise with how you move. The sheer amount of control you have now over Sonic while in the air allows for a lot of creativity when considering how to traverse a level, and this is in a lot of ways a requirement for getting past some of the harder parts of the game.
There has been a lot of effort thrown into the levels to try to keep them varied and unique with respect to each other. While for some this may mean holdups as you try to figure out how to solve certain obstacles, a few tries will see you racing past them at full speed. With that speed though comes lots of heart-attack-inducing moments as you barely skim past immediate death quite often. For many, this is pretty awesome and really gives a huge rush as you play through the game, though it may also frustrate others. I personally enjoyed the raw fear induced by trying to race through parts of the second labyrinth level in near complete darkness, and the moments of relief when you happen to light one of the torches and get a better view of things. The levels requiring constant forward movement, such as the last stage of the 4th zone, are equally panic-inducing, but a lot of fun. In the midst of this are also more relaxed levels such as those in the casino street zone, where lives are abundant and throwbacks to the casino night zone of sonic 2 make their return. The bonus stages of old also make their return, and their cruel harshness towards small errors is as prominent as ever, but still results in a lot of fun and a huge sense of satisfaction when you finally get a level's emerald. The bosses in this game are sometimes a little too similar to those of the previous games, but are still enjoyable nonetheless. The main complaint of course is that there are so few levels to the game, but then again, this is only episode 1.
Graphics-wise, a side-scrolling sonic game has never looked better. The artists have really gone out of their way to portray the old-school sonic style with a well-deserved modern clean-up. Back are the familiar checkerboards and bright colors of old, though with appropriate depth now added thanks to the 3d modeling. The music also is reminiscent of old, though maybe a little less catchy. The sound effects have also retained their classic style that many will remember from so long ago. In fact, were it not for the slight depth added by the 3d modeling, this would look and sound exactly as if it were made shortly after sonic and knuckles.
As a whole, sonic 4 episode 1 is a great game with a lot of enjoyment in store. While it may take some getting used to for older fans of the series, there are a lot of new challenges and great moments to be had while still paying proper homage to its predecessors. If this is the road that sonic is now taking in the side-scrolling world, then there is a lot to look forward to in the coming episodes.